Multiple users write to the same file at the same time using PHP

Question

I have a website where at the same moment, there can be multiple users writing to the same file at the same time. An example of my code is below.

PHP 5.6

Answer 1

This is a common theoretical problem for many web applications in many programming languages.

The answer is yes, it CAN cause trouble. However, this is a very theoretical problem, if the contents you are adding to the files aren't very big and if you don't have heavy traffic. Today's operating systems and file systems are so well optimized (Caching, lazy writing etc.) that it is very unlikely to happen, when you close your file handles immediately after using them.

You could add something like a buffer, if you run into an error (check access rights before writing with PHP/catch exceptions in other languages) and try again after some delay or write your buffer to a temp file and merge it with another process - you have several possibilities.

And yes, flock() is a function that could be good for these purposes, but I think, that would be already over-engineered.

Answer 2

You need to change saving mechanism from simple file into any database with transactions.

Answer 3

If two scripts attempt to write to a file at the same time. The fopen() function, when called on a file, does not stop that same file from being opened by another script, which means you might find one script reading from a file as another is writing, or, worse, two scripts writing to the same file simultaneously. So it is good to use flock() . You can get more help on http://www.hackingwithphp.com/8/11/0/locking-files-with-flock . For your code you may use flock() as

<?php
$root=realpath($_SERVER["DOCUMENT_ROOT"]);
$var=time()."|";
echo $var."<br/>";
$filename=$root."/Testing/Testing/test1.txt";

$myfile=fopen($filename,"a");
if (flock($myfile, LOCK_EX)) {        
    fwrite($myfile,$var);
    flock($myfile, LOCK_UN); // unlock the file
} else {
    // flock() returned false, no lock obtained
    print "Could not lock $filename!\n";
}

fclose($myfile);

$myfile=fopen($filename,"r");
if (flock($myfile, LOCK_EX)) {        

    $contents = fread($myfile, filesize($filename));
    echo $contents;
    flock($myfile, LOCK_UN); // unlock the file
} else {
    // flock() returned false, no lock obtained
    print "Could not lock $filename!\n";
}
fclose($myfile);
?>

Answer 4

I'm not familiar with flock, but my first thought is a locking or queueing mechanism. If the data written does not have to be used or view back to the users, then a work queue would be the best choice. Write the data to a redis or memcached based system, sql or another type of queueing system, or just dump unique timestamped files with the interesting content in a directory that a worker can aggregate in ascending order into the maste file.

For use cases where the written data triggers something the user need to get a report or result of, respond to, or other feed back, then locking might be the way if you can't re-architect to an async stack with eventual consistency. It's also difficult to know with out knowing the load and number of concurrent users, the number of servers etc.